Jaguar XJS Won't Crank, Starter Relay
On the XJS the relay and the starter both fail often, and one jumper wire tells them apart: key at II, 12 V on the white/red wire from the relay to the solenoid. If it cranks, it's the relay, its earth through the gear selector or the ignition switch; if not, the starter or its heavy feed from the firewall posts. Instruments, radio and ignition dead together while lights and hazards work: the LB202 connector under the dash.
How bad is it · Garage, carefully
Stranded — and sometimes a fire risk
An overheating LB202 connector can cut the ignition while driving, and a starter that stays engaged or a shorted starter feed can smoke and melt insulation. If you smell burning, disconnect the battery.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
The starter relay
The large silver relay under a plastic cover at the right rear of the engine bay switches the solenoid. Its contacts wear and pit, and a new one can be faulty out of the box.
A click or nothing from the key, yet 12 V on the white/red wire cranks it at once. A few taps on the relay sometimes bring it back.
Key at II, 12 V on the white/red wire — the relay's output to the solenoid. If it cranks, it's the relay.
Replace the relay; a generic 30 A automotive relay wired to the same terminals does the job. A few blows with a rubber mallet can get you going until it arrives.
The relay's earth through the gear selector
The relay coil is earthed only through the Park/Neutral switch and its inhibit relay. A worn contact or a bad earth there, and the relay never pulls in — a new relay or ignition switch changes nothing.
No click at all while everything else lights up; moving the lever or the wires at the selector sometimes lets it crank.
In Park, measure the relay's earth wire (white/black) to ground. If it's open, follow it to the selector switch and inhibit relay.
Restore the earth: repair the wiring at the gear-lever safety switch, or replace a worn switch.
The starter itself
The XJS starter lives next to the hot exhaust. Heat, oil and age kill it: magnets crack, commutator segments lift, internal earths fail, solenoid contacts weld.
12 V on the white/red wire and still no crank; a single clean click; trouble only when hot; headlamps dimming only slightly.
Confirm voltage at the solenoid while cranking. A hot no-start that goes away cold points at the starter.
Originals are scarce, so fit a gear-reduction starter — and refit the heat shield.
The firewall power posts and the heavy cable to the starter
Battery power from the boot reaches the engine bay on two firewall posts, each with two nuts and several terminals under a rubber boot. Corrosion there, or at a splice in the starter cable, starves the starter while the lights work.
A jump helps once, then not; a missing boot on a post; heat marks on a nut.
Battery off first — the posts are always live. Undo both nuts on each post, clean every terminal and inspect the cable to the starter.
Clean the posts and replace a heat-damaged cable — the bus bar cable, or the one from the right-hand post to the starter; repair a corroded splice near the gearbox dipstick.
LB202: the dash, radio and ignition die together
LB202, a black four-way connector under the dash right of the steering column, carries battery power to the ignition switch and back to the load relay. Its brown and white contacts overheat and the ignition circuit drops out.
The car dies on the move, then cranks but won't fire: instruments, warning lamps, radio and trip computer dead; lights, starter and hazards working. Early warning: the voltmeter dips and the rev counter flicks when you brake.
Remove the lower dash and look for heat damage on the brown and white terminals.
Replace or refinish the contacts, or cut out the plug and join the wires. Cleaning alone can fail again within a quarter of an hour.
Relays and switches in the chain: corroded blades, the inertia switch wiring
Several relays and the crash switch sit in the start chain. Corroded relay blades, or wiring damaged at the inertia switch, stop the start signal.
Intermittent, often from cold; no fuel pump prime at key-on as well as no crank.
Pull and clean the relays in the engine-bay rows; check the wiring at the inertia switch and press its button.
Clean corroded relay blades, for example in the passenger-side row, and repair wires cut through by the inertia switch cover.
A short on the starter feed, or added wiring
The starter feed is always live. A dropped part, worn insulation or badly wired accessories can short it or pull it down.
Smoke near the gearbox dipstick; a huge spark on reconnecting the battery; trouble that began with added fog lamps.
Take the heavy starter wire off the right-hand post and test for a short to earth; disconnect added accessories.
Remove whatever is touching the starter terminal, repair the shorted feed, or disconnect the badly wired accessory.
The battery
A V12 needs a strong battery. Rapid clicking with the headlamps dimming hard is a battery that can't hold up.
Rapid clicking, lights dimming hard; a battery that has to live on a charger.
Check the electrolyte if you can, and load-test.
Top up dry cells with distilled water and recharge, or replace a tired battery.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Watch the headlamps as you turn the key
Heavy dimming and rapid clicking: battery or heavy connections. No dimming: the signal isn't reaching the starter.
freeKey at II, 12 V on the white/red wire
At the starter relay, right rear of the engine bay. Cranks: relay, its earth or the ignition switch. Doesn't: the starter or its feed.
a jumper wireTry Neutral and move the lever
Then measure the relay's earth wire in Park.
freeClean both firewall posts
Battery off; both nuts on each post.
freeInstruments died too? Check LB202
Under the dash right of the column; look for heat on the brown and white contacts.
freeThen the starter
Downpipe off for access; keep the heat shield.
gear-reduction unitWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS V12 Won't Start or Starts and DiesIf the engine turns over and does not start.
- XJS fuse boxes and relaysOther years from the same page.
- Jaguar XJS Air Conditioning ProblemsIf the A/C clutch won't engage: its relay sits on the same right-hand wing, checked the same way.
- Jaguar XJS 3.6 and 4.0 Won't StartIf it turns over normally but won't fire on the 3.6 or 4.0 six-cylinder.
- Jaguar XJS Electrical Problems, Bad EarthsIf other circuits misbehave too, or wiring smokes while cranking: the engine earth and fuse boxes.
- Jaguar XJS Transmission Problems, GM400 and ZF GearboxIf the lever positions don't match the gearbox or it cranks only in Neutral, see the transmission page.
- Jaguar XJS Alarm Keeps Going OffIf the alarm went off before it stopped cranking: the security module holds the starter enable.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.